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冷泉是指来自海底沉积界面之下,与海水温度相似,以水、碳氢化合物(天然气和石油)、硫化氢、细粒沉积物为主要成分的流体以喷涌或渗漏方式从海底溢出的渗漏活动(Paull et al.,1984; Suess et al.,1985)。其广泛发育于大陆活动和被动大陆边缘海底,是继洋中脊热液被发现和研究之后又一个最令人兴奋的新发现之一,为科学研究开辟了一个新领域(Paull et al.,1984; Suess et al.,1985; Peckmann et al.,2001; Peckmann et al.,2004; Bayon et al.,2011, 2013)。1983年,美国科学家Charles Paull首次在加利福尼亚州蒙特利尔海湾(Monterey Bay)水深3200m的海底,发现了富含天然气和硫化氢的流体,该流体的温度与周围海水接近,为了区别于海底热液,Charles Paull将其命名为冷泉(Cold seepage) (Paull et al.,1984)。自此,在全球范围内便不断有学者对海底冷泉进行了报道。到目前为止,对海底冷泉的研究已经持续了近40多年,全世界共发现有900多处冷泉(陈多福等, 2002; Feng Dong et al.,2018; Suess, 2020)。并且随着研究的深入,学者们对海底冷泉系统也有了更深刻和全面的认识。
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冷泉系统由于与天然气水合物、极端环境生物活动和全球变暖等重大科学问题密切相关而成为当前国际科学前沿热点之一(卢苗安等, 2002; DeConto et al.,2012; Crémière et al.,2016; Ruppel et al.,2017)。研究表明,冷泉可能与海底沉积物中天然气水合物的分解有关,是探索海底天然气水合物资源的重要指标,对水合物资源的探索和开发具有重要的指示意义(Feng Dong et al.,2014, 2015a, 2015b, 2018)。同时,冷泉活动伴随大量的甲烷释放,其中一部分能够直接进入海水甚至大气 (Boetius et al.,2013; Ruppel et al.,2017)。甲烷是比二氧化碳威力更强的温室气体,大量甲烷进入大气中必然会加速全球气候变暖(Kennett et al.,2000; Chen Fang et al.,2019; Wei Jiangong et al.,2020, 2022; Deng Yinan et al.,2021)。但事实上,在甲烷渗漏过程中,有超过90%的甲烷受到由嗜甲烷古菌和硫酸盐还原菌所介导的甲烷厌氧氧化作用(Anaerobic oxidation of methane, AOM),在硫酸盐-甲烷转换带内(Sulfate-methane transition zone, SMTZ)被消耗(Boetius et al.,2000; Hill et al.,2004, 2012; Joseph et al.,2013)。化学方程式为:CH4+SO2-4→2HCO-3+HS-+H2O。这一过程不仅有效阻止了甲烷向大气中的排放,同时反应过程中生成的硫化氢能够为冷泉生物提供能量,维系着以化能自养生物为食物链基础的冷泉生态系统(Peckmann et al.,2001; Feng Dong et al.,2014, 2015a, 2015b, 2018; Suess, 2014, 2018)。一直以来深海环境几乎是“海底沙漠”的代名词,那里阳光无法到达,光合作用不能进行,许多生物缺乏生存必需的食物来源。但冷泉生态系统的发现打破了“万物生长靠阳光”这一定律,因此冷泉区被也认为可能是生命的起源地之一,对于研究早期地球的生命起源与演化具有重要意义。此外,由于AOM产生了大量的HS-,海洋的氧化还原条件也会发生变化。研究表明,地质历史时期的诸多海洋缺氧事件的幕后推手之一,即为水合物分解形成的甲烷渗漏事件,如诛罗纪早托尔阶大洋缺氧事件(Early Toarcian OAE)(Hesselbo et al.,2000)、白垩纪阿普特阶大洋缺氧事件(Aptian OAE)(Grcke et al.,1999)。
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冷泉碳酸盐岩的形成与冷泉密切相关,它是海底冷泉活动的主要产物之一(Peckmann et al.,2004; Bayon et al.,2007, 2011, 2013)。自Suess et al.(1985)在美国俄勒冈州首次发现与冷泉有关的自生碳酸盐岩以来,科学家已在全球范围内众多大陆边缘发现了冷泉及冷泉碳酸盐岩(图1a)。冷泉碳酸盐岩很好地保存了冷泉形成演化过程的地球化学特征,记录了冷泉活动的历史,可为探讨冷泉系统的活动和演化、流体来源和运移过程以及沉积环境等提供信息。因此,通过冷泉碳酸盐岩可以更好地了解古冷泉活动信息,重建区域古冷泉演化过程(Peckmann et al.,2001; Peketi et al.,2012, 2015; Hu Yu et al.,2014, 2015, 2017; Crémière et al.,2016; Deng Yinan et al.,2021; Wei Jiangong et al.,2022)。
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我国对冷泉碳酸盐岩的研究可追溯到20世纪90年代。广州海洋地质调查局对南海的天然气水合物调查过程中,科学家在南海北部陆坡发现了多处的水合物异常区,并获取了大量冷泉碳酸盐岩样品(图1b;邓希光等,2008;蒲晓强等,2009)。目前有诸多对冷泉碳酸盐岩的矿物学和地球化学特征的研究(Hu Yu et al.,2014, 2015, 2017; Feng Dong et al.,2018; Deng Yinan et al.,2021; Wei Jiangong et al.,2022),但仍有一些问题尚未解决。本文总结了近年来南海北部冷泉碳酸盐岩的研究成果和存在的问题,并进一步提出了日后的研究方向。
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1 地质背景
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南海作为西太平洋最大的边缘海之一,是在亚欧板块、太平洋板块和印澳板块的共同作用下形成的(Taylor et al.,1980)。由于三大板块的相互作用,使得南海地壳受到多方面的构造应力作用,形成了独特的地球物理场和边界构造特征。南海形成与演化过程中经历了三次大规模的构造运动,致使其具有良好的天然气水合物形成和保存的优良环境。其中在第三次扩张活动的沉积作用下,南海输入了大量富含有机质的陆源物质,沉积速率较高,为水合物形成提供充足的气源(吴必豪等,2003)。南海海底合适的稳温压条件进一步为水合物成藏提供了便利(王敏芳,2003;刘建章等,2004;陆红锋等,2011)。南海陆缘区在为水合物形成提供充足的气源以及适宜的温压场环境的同时,还为水合物形成提供了有利的构造条件。南海独特的海底地貌,包括海台、海槽、陡坎、泥底辟等,也有利于烃类气体的运移(吴必豪等,2003;陈多福等,2004;王秀娟等,2008;陆红锋等,2011;Hui Gege et al.,2016)。因此,南海是研究冷泉活动的天然实验室。
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2 南海北部冷泉碳酸盐岩矿物学和地球化学特征及其指示意义
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在冷泉活动过程中,AOM的广泛发育生成了大量的HCO-,导致周围环境碱度增高。同时,HCO-与海水中的Mg2+、Ca2+、Sr2+等阳离子结合进一步形成了自生碳酸盐岩沉淀(Peckmann et al.,2001, 2004),化学方程式为:Ca2++2HCO-3→CaCO3+CO2+H2O。
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图1 全球冷泉站位分布示意图(a)(据Suess,2014修改)和南海北部冷泉碳酸盐岩站位分布示意图(b) (据Han Xiqiu et al., 2008, 2014; 陆红锋等, 2011; Tong Hongpeng et al.,2013; Feng Dong et al.,2015a, 2015b, 2018; Li Niu et al.,2016; Liang Qianyong et al.,2017; Yang Kehong et al.,2018; Wei Jiangong et al.,2020;2022;Deng Yinan et al.,2021修改)
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Fig.1 Schematics showing global distribution of cold seeps (a) (modified after Suess, 2014) and schematics showing cold seep carbonates distribution in the northern South China Sea (b) (modified after Han Xiqiu et al.,2008, 2014; Lu Hongfeng et al.,2011; Tong Hongpeng et al.,2013; Feng Dong et al.,2015a, 2015b, 2018; Li Niu et al.,2016; Liang Qianyong et al.,2017; Yang Kehong et al.,2018; Wei Jiangong et al.,2020, 2022; Deng Yinan et al.,2021)
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(a)图中蓝色正方形代表主动大陆边缘海底冷泉,黄色正方形代表被动大陆边缘海底冷泉,绿色正方形代表转换大陆边缘海底冷泉; (b)图中绿色圆形代表表层碳酸盐岩,蓝色五角星代表钻探获取的深部碳酸盐岩
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In Fig.1(a), the blue squares represent cold seep around active continental margin, the yellow squares represent cold seep around passive continental margin, the green square represent cold seep around transform continental margin.In Fig.1(b), the green circles represent the surface carbonates, the blue pentacles represent deep carbonates that have been drilled
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受冷泉活动过程中的甲烷通量、流体渗漏速率以及生物扰动等因素的影响,冷泉碳酸盐岩往往具有独特的矿物学和地球化学特征,被认为是记录冷泉活动过程信息的优良载体(Feng Dong et al.,2016; Chen Fang et al.,2021; Jin Meng et al.,2021)。科学家们藉此对地质历史时期的甲烷渗漏活动进行了综合研究。
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2.1 矿物学特征
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南海北部海底冷泉区出露大量碳酸盐岩,通常以结壳状、结核状、烟囱状、角砾状、块状等不同的形态产出(Han Xiqiu et al.,2008, 2014; Tong Hongpeng et al.,2013; Feng Dong et al.,2015a, 2015b, 2016, 2018; Chen Fang et al.,2019; Ge Lu et al.,2020; Wei Jiangong et al.,2020, 2022; Deng Yinan et al.,2021)。不同区域碳酸盐岩矿物组成具有明显的差异性,其中文石和高镁方解石在南海北部最常见,琼东南海域、西沙海槽海域、东沙海域、台西南海域均有分布(Tong Hongpenget al.,2013; Han Xiqiuet al.,2014; Feng Dong et al.,2018; Ge Luet al.,2020; Deng Yinan et al.,2021; Wei Jiangong et al.,2022);白云石则只出现在神狐海域、东沙海域的少数站位(Han Xiqiu et al.,2008; 陆红锋等, 2011)。研究表明,甲烷渗漏通量的变化导致了冷泉碳酸盐岩矿物相的变化。在强甲烷渗漏活动中,较浅的SMTZ导致碳酸盐岩形成于海底表层或接近浅层的相对开放的环境中(Feng Dong et al.,2016; Lin Zhiyong et al.,2016; Chen Fang et al.,2021),此处充足的SO2-4供应会抑制Mg2+进入晶格,导致碳酸盐析出以文石为主(Burton, 1993; Goetschl et al.,2019)。而在低甲烷渗漏的深部相对封闭环境中,SO2-4含量较低且补充不及时,这有利于方解石的形成(Goetschl et al.,2019; Chen Fang et al.,2021)。
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2.2 氧碳同位素特征
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冷泉碳酸盐岩形成过程受甲烷等烃类的厌氧氧化作用影响较大,因而会继承其碳氧同位素特征(Sackett, 1978; Whiticar, 1999; Peckmann, 2004; Feng Dong et al.,2018; Deng Yinan et al.,2021)。因此,通过分析碳酸盐岩碳氧同位素组成可以判别其形成过程中的流体性质。自然界中甲烷主要有两种成因:生物成因和热解成因。生物成因甲烷的δ13C值范围为-110‰~-50‰(Whiticar, 1999),热解成因甲烷的δ13C值范围为-50‰~-30‰(Sackett, 1978)。此外,由于海水(δ13C值约为0‰)的混染作用 (Feng Dong et al.,2018),最终形成的碳酸盐岩δ13C值变化范围较大(图2)。研究发现,南海北部自生碳酸盐岩主要为甲烷成因,并且其沉淀流体碳源在西南部以热解成因气为主,而在东北部则以生物成因气为主(Han Xiqiu et al.,2008, 2014; Tong Hongpeng et al.,2013; Feng Dong et al.,2015a, 2015b; Liang Qianyong et al.,2017; Wei Jiangong et al.,2020, 2022)。但是由于冷泉碳酸盐岩中碳的来源具有复杂性,包括海水碳源、甲烷碳源以及产甲烷残留碳源等都会对冷泉碳酸盐岩中碳同位素组成产生影响(Feng Dong et al.,2018),因此要科学合理利用δ13C进行甲烷成因的判别。
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南海的冷泉活动通常与水合物分解有关(Han Xiqiu et al.,2014; Feng Dong et al.,2015a, 2015b, 2018; Wei Jiangong et al.,2020, 2022),通过碳酸盐岩的氧同位素值也可以判断其流体的性质。在水合物生成过程中,甲烷优先与含18O的水分子结合,这导致水合物晶格中的δ18O值要高于周围的水体(Bohrmann et al.,1998; Feng Dong et al.,2018)。因此,当水合物分解时,富集18O的流体更多的参与碳酸盐岩的形成过程,导致其δ18O值较高(Hu Yu et al.,2012, 2014)。但黏土矿物的脱水作用同样能够导致流体中富集18O(Hesse, 2003)。因此,在利用碳酸盐岩氧同位素判别其形成过程中流体来源时,要进行相应的排除。
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2.3 氧化还原敏感元素和稀土元素特征
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由于冷泉碳酸盐岩是甲烷厌氧氧化作用的产物,传统观点认为其应形成于厌氧环境中(Hu Yu et al.,2015; Feng Dong et al.,2018)。在南海科学家利用氧化还原敏感元素以及稀土元素对冷泉碳酸盐岩的形成环境进行了重建。根据U和Mo在不同水体环境下富集机制的差异,Mo-U富集系数比(MoEF/UEF)被广泛应用于氧化还原环境的重建(Algeo et al.,2009; Chen Fang et al.,2016; Miao Xiaoming et al.,2021a)。由图3可以看出,冷泉碳酸盐岩中MoEF/UEF值大多大于1×SW (Sea Water),这表明冷泉环境主要为还原环境,但仍有部分MoEF/UEF值小于0.3×SW,显示为氧化环境(Liang Qianyong et al.,2017; Deng Yinan et al.,2021; Wei Jiangong et al.,2022)。同时,研究者在南海冷泉碳酸盐岩中发现了不同类型的Ce异常特征(图4)。既有指示缺氧环境的Ce正异常和无异常特征,又有指示氧化环境的Ce负异常特征(Tong Hongpeng et al.,2013; Feng Dong et al.,2015a, 2015b; Deng Yinan et al.,2021; Wei Jiangong et al.,2022)。结合碳酸盐岩MoEF/UEF和Ce正异常特征,本文认为南海冷泉碳酸盐岩大多形成于还原环境,也有部分可能存在于有氧环境。但碳酸盐岩在从水体中沉积的过程中,不可避免的会混入硅酸盐矿物等非碳酸盐岩组分,且碳酸盐岩也易受到后期成岩作用的改造(Zhao Yanyan et al.,2021),这些都会对原始海洋沉积环境的恢复造成干扰。因此,在运用碳酸盐岩进行古冷泉环境恢复时,需要保证其能够反应原始沉积环境的有效信息。其中Ce/Ce*与LaN/SmN和DyN/SmN以及LaN/SmN和∑REE比值的相关性得到了广泛的应用(Tong Hongpeng et al.,2013; Zhao Yanyan et al.,2021; Wei Jiangonget al.,2022)。
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图2 南海陆坡不同海域碳酸盐岩氧碳同位素特征(据Han Xiqiu et al.,2008, 2014; Tong Hongpeng et al.,2013; Feng Dong et al.,2015a, 2015b; Li Niu et al.,2016; Liang Qianyong et al.,2017; Yang Kehong et al.,2018; Deng Yinan et al.,2021; Wei Jiangong et al.,2022修改)
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Fig.2 Carbon and oxygen isotopes in the seep carbonates in different seas on the continental slope of the South China Sea (modified after Han Xiqiu et al.,2008, 2014; Tong Hongpeng et al.,2013; Feng Dong et al.,2015a, 2015b; Li Niu et al.,2016; Liang Qianyong et al.,2017; Yang Kehong et al.,2018; Deng Yinan et al.,2021; Wei Jiangong et al.,2022)
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图3 南海陆坡不同海域Mo-U富集系数比值图(据Liang Qianyong et al.,2017; Yang Kehong et al.,2018; Deng Yinan et al.,2021; Wei Jiangong et al.,2022修改)
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Fig.3 Mo-U enrichment factor ratio diagram in different seas on the continental slope of the South China Sea (modified after Liang Qianyong et al.,2017; Yang Kehong et al.,2018; Deng Yinan et al.,2021; Wei Jiangong et al.,2022)
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图4 南海陆坡不同海域Ce/Ce*-Pr/Pr*示意图(据Han Xiqiu et al.,2014; Feng Dong et al.,2015a, 2015b; Yang Kehong et al.,2018; Deng Yinan et al.,2021; Wei Jiangong et al.,2022修改)
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Fig.4 Schematic diagram of Ce/Ce*-Pr/Pr* in different seas on the continental slope of the South China Sea (modified after Han Xiqiu et al.,2014; Feng Dong et al.,2015a, 2015b; Yang Kehong et al.,2018; Deng Yinan et al.,2021; Wei Jiangong et al.,2022)
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3 目前存在的问题及未来展望
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本文对南海冷泉碳酸盐岩的研究现状进行了综合分析,笔者认为虽然目前的研究取得了一些重大成果和突破,但可能仍存在一定的局限性:① 在研究内容上,科学家主要关注冷泉碳酸盐岩的成岩过程及其环境影响,包括流体来源、氧化还原环境、元素循环等(Hu Yu et al.,2015; Feng Dong et al.,2018),而有关冷泉活动对全球变暖和碳循环的影响研究很容易被忽略;② 受限于取样方式,南海的冷泉碳酸盐岩多为浅表层的样品,缺乏较深层位的样品。这导致目前冷泉碳酸盐岩的研究具有单一性,更多的研究主要集中于单一期次,南海冷泉活动的历史重建仍存在很多挑战(Tong Hongpeng et al.,2013; Han Xiqiu et al.,2014; Feng Dong et al.,2018)。因此,本文认为对海洋沉积物中深部地层中保存的碳酸盐岩进行综合研究具有重要意义。
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结合南海北部碳酸盐岩的形成年龄(基于U-Th测年数据)进行综合分析(图5),可以更好地了解南海地质历史时期冷泉活动过程。以往研究表明,南海冷泉活动主要发生于低海平面时期或者冰期向间冰期的过渡时期(Feng Dong et al.,2010; Tong Hongpeng et al.,2013; Han Xiqiu et al.,2014; Yang Kehong et al.,2018; Wei Jiangong et al.,2020, 2022; Deng Yinan et al.,2021),这是因为海底天然气水合物是低温高压环境下的产物,极易受周边环境的影响。当海底压力降低或者底层水温升高都会导致水合物的分解,随后产生大量的甲烷渗漏(Deng Yinan et al.,2021; Wei Jiangong et al.,2022)。在低海平面时期大多为冰期,在这一时期气候较为寒冷,并不利于水合物的分解。然而该时期海平面高度明显下降,能够极大的降低海底水合物的上覆压力,致使水合物发生分解(Feng Dong et al.,2018; Miao Xiaoming et al.,2021a)。与之相反,发生于冰期向间冰期的过渡时期的冷泉活动则与海平面变化关系不大。在这一时期,海平面迅速升高,抑制了水合物的分解。但诸多研究表明,在冰期到间冰期的过渡期间,几乎所有海域均出现了底水温度升高的现象(Rohling et al.,2014)。例如,在冰消期,大西洋底水温度增加了3~4.5℃(Dwyer et al.,1995);南海北部陆坡底水温度增加了1.8~4.5℃(Chen Fang et al.,2019)。而且底层水温度每升高1℃,就足以触发局部水合物的分解(Reagan et al.,2007)。因此,笔者认为当南海发生底层水温的升高现象时,最终会导致水合物的分解和随后的甲烷渗漏。
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根据上述分析,本文研究认为需要进一步挑选合适的地点进行南海冷泉活动重建工作。这些地点需要具备以下几个条件:① 海底具有丰富的水合物矿藏;② 历史上或目前发生过冷泉活动事件。实际上,近年来科学家在琼东南海域和东沙海域冷泉区已经进行了一些初步的工作,在不同深度层位中的沉积物样品中发现了冷泉碳酸盐岩(Chen Fang et al.,2019; Wei Jiangong et al.,2020, 2022; Deng Yinan et al.,2021)。测年结果显示发现其主要形成于约130ka以来的冰期向间冰期转变的过渡时期(Chen Fang et al.,2019; Wei Jiangong et al.,2020, 2022; Deng Yinan et al.,2021)。同时,科学家进一步讨论了冷泉活动与全球气候变化和碳循环之间的关系,使我们能够从多方面更好地了解全球气候变化和碳循环(Chen Fang et al.,2019; Wei Jiangong et al.,2020, 2022; Deng Yinan et al.,2021)。此外,琼东南海域和东沙海域也是南海现存的两处冷泉活动区(Feng Dong et al.,2015a, 2015b; Liang Qianyonget al.,2017)。因此,琼东南海域和东沙海域冷泉区是未来可以通过大洋钻探进行地质历史时期冷泉活动研究的理想区域。
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图5 世界各海域中冷泉碳酸盐岩年龄和深海氧同位素阶段示意图(据Watanabe et al.,2008; Feng Dong et al.,2010, 2015a, 2015b; Tong Hongpeng et al.,2013; Han Xiqiu et al.,2014; Li Niu et al.,2016; Liang Qianyong et al.,2017; Yang Kehong et al.,2018; Himmler et al.,2019; Deng Yinan et al.,2021; Wei Jiangong et al.,2022修改)
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Fig.5 Ages of seep carbonates versus marine isotope stages in different seas of the world (modified after Watanabe et al.,2008; Feng Dong et al.,2010, 2015a, 2015b; Tong Hongpeng et al.,2013; Han Xiqiu et al.,2014; Li Niu et al.,2016; Liang Qianyong et al.,2017; Yang Kehong et al.,2018; Himmler et al.,2019; Deng Yinan et al.,2021; Wei Jiangong et al.,2022)
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受限于研究素材,南海冷泉碳酸盐岩记录的甲烷渗漏活动仅仅可以追溯到三十几万年左右(Feng Dong et al.,2018)。此时段内,南海构造活动相对平稳,海平面和水温变化是控制水合物分解的主要因素。但地质历史时期南海构造活动活跃,断层、底劈等频繁发生,极有可能曾引发更大规模的水合物分解。因此,海平面和水温变化控制了南海水合物分解这一结论有一定的局限性。希望通过南海大洋钻探项目的开展,能够为探讨南海演化与冷泉活动的相关关系提供基础资料。
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摘要
冷泉系统在大陆边缘海域中广泛发育,我国的南海北部海域因其独特的地质条件成为了研究冷泉系统的天然实验室。在冷泉系统中,由于甲烷厌氧氧化作用的存在,极大地促进了碳酸盐岩等自生特征矿物的形成。而自生碳酸盐岩继承了冷泉环境中的地球化学特征,是记录冷泉活动信息的优良载体。近十几年以来,国内外科学家对南海北部冷泉碳酸盐岩进行了大量的研究,取得了一系列的成果,但有关南海北部碳酸盐岩最新研究成果的整体认识和未来的研究方向仍缺乏相应的汇总分析。因此,本文对近年来南海北部冷泉碳酸盐岩的相关研究成果进行了综述。目前研究主要关注冷泉碳酸盐岩的成岩过程及其环境影响,包括流体来源、氧化还原环境、元素循环等,而有关冷泉活动对全球变暖和碳循环影响的研究较少;同时大多研究基于单一期次的冷泉活动,有关地质历史时期的冷泉活动鲜有报道。本文在对已有研究基础上进行综述,认为未来需要在南海北部合适的区域进行大洋钻探,重建地质历史时期以来的冷泉活动,探讨其对全球变暖和碳循环的影响。基于前期的工作成果,笔者认为琼东南海域和东沙海域冷泉区是对地质历史时期冷泉活动进行深入研究的理想区域。
Abstract
Cold seep systems are widely developed in the continental margins. Among them, the northern South China Sea has become a natural laboratory for the study of cold seep because of its unique geological conditions. In cold seep systems, the existence of anaerobic oxidation of methane greatly promotes the formation of authigenic minerals such as carbonates. Authigenic carbonates inherit the geochemical characteristics of cold seep environment, so they are considered as good proxies for recording the activity of cold seep. In recent years, scientists have done a lot of research on cold-seep carbonates in the north of the South China Sea, and a series of achievements have been made. However, the overall understanding of the latest research results and the future research direction of the carbonates in the northern South China Sea are still lacking in the absence of a corresponding summary analysis. Therefore, this paper reviews the related research results of cold-seep carbonates in the northern South China Sea. At present, most scientists mainly focus on the diagenetic process of cold-seep carbonates and its environmental influence, including fluid source, redox environment, element cycle, etc., but the influence of cold seep activity on global warming and carbon cycle is less discussed. At the same time, most studies are based on a single period of cold seep activity, and there are few reports about the cold seep activity in geological history. Therefore, to address these issues, we need to conduct ocean drilling in suitable areas in the northern part of the South China Sea to reconstruct cold seep activity in geological history and to explore its impact on global warming and the carbon cycle. In addition, based on previous work, we believe that the cold seep area in the Qiongdongnan area and the Dongsha area is the ideal area for the future study of cold seep activity in geological history.
Keywords
natural gas hydrate ； methane seepage ； seep carbonate ； South China Sea
